1. Field of the Invention
The present invention relates to a method and apparatus for extruding a laminated film, and particularly a tubular laminated film in which each of the polymer layers in the film is optimally molecularly oriented to impart desired structural properties to the film. More particularly, the invention relates to a method and apparatus for forming a laminated film, particularly a tubular laminated film, containing at least one properly formed foamed polymer layer and in which each of the polymer layers is optimally molecularly oriented.
2. Discussion of the Prior Art
It is often desirable to co-extrude different polymers to form a laminated film in order to impart desired strength or other physical properties to the film. Often, the extruded polymers which form layers in the laminated film have considerably different viscosities and viscoelastic behavior properties. This is true, for example, of high density and low density polyethylene. When two dissimilar polymer layers are joined within an extrusion die, the resulting extruded laminated film tends to assume the properties of the "weaker" layer so that subsequent blown expansion of the extruded film optimizes the molecular orientation properties of the weaker polymer layer, but not the other. For example, high density polyethylene typically requires a high blow-up ratio for optimal orientation, while low density polyethylene requires a lower blow-up ratio. When these two polymer materials are co-extruded, the molecular orientation imparted to both layers during blown expansion is closer to the orientation desired for the low density polyethylene and the high density polyethylene layer is not optimally oriented.
Accordingly, when polymers having different viscosities and viscoelastic behavior properties are joined and co-extruded through a tubular die, different flow path configurations, tailored to each polymer, must be provided within the die to ensure that the polymer layers in the subsequently extruded and blown laminated film will be properly molecularly oriented to achieve a desired orientation property for the film as a whole. Providing flow paths specifically tailored to a polymer is both time consuming and expensive and requires the construction of unique dies for extruding particular polymer layer combinations.
In addition, when one of the extruded polymer layers is a foamed polymer and the other is not, it is difficult to obtain a proper foaming and foam cell shape in the extruded layer because it exits the die laminated to a dissimilar polymer layer which affects the polymer foaming operation. In other words, when extruding a polymer foam layer, a particular shear rate history is required as the polymer exits the die orifice to ensure proper foaming, but this is difficult to achieve when the foamed layer is co-extruded as part of a laminated film. The co-extrusion of a foamed polymer layer with another dissimilar polymer could produce cells in the foamed layer which are excessively elongated, producing a foamed layer which easily fractures and splits.